The present exemplary embodiments relate to phosphor compositions for the conversion of radiation emitted by a light source. It finds particular application in conjunction with converting LED-generated ultraviolet (UV) or blue radiation into white light for general illumination purposes. It should be appreciated, however, that the invention is also applicable to the conversion of radiation from UV and/or blue lasers as well as other UV sources to white light.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. They are particularly useful as display lights, warning lights and indicating lights or in other applications where colored light is desired. The color of light produce by an LED is dependent on the type of semiconducting material used in its manufacture.
Colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been produced from Group III-V alloys such as gallium nitride (GaN). To form the LEDs, layers of the alloys are typically deposited epitaxially on a substrate, such as silicon carbide or sapphire, and may be doped with a variety of n and p type dopants to improve properties, such as light emission efficiency. With reference to the GaN-based LEDs, light is generally emitted in the UV and/or blue range of the electromagnetic spectrum. Until quite recently, LEDs have not been suitable for lighting uses where a bright white light is needed, due to the inherent color of the light produced by the LED.
LEDs rely on its semiconductor to emit light. The light is emitted as a result of electronic excitation of the semiconductor material. As radiation (energy) strikes atoms of the semiconductor material, an electron of an atom is excited and jumps to an excited (higher) energy state. The higher and lower energy states in semiconductor light emitters are characterized as the conduction band and the valence band, respectively. The electron, as it returns to its ground energy state, emits a photon. The photon corresponds to an energy difference between the exited state and ground energy state, and results in an emission of radiation.
Recently, techniques have been developed for converting the light emitted from LEDs to useful light for illumination purposes. In one technique, the LED is coated or covered with a phosphor layer. A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range.
By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum may be generated. Colored LEDs are often used in toys, indicator lights and other devices. Manufacturers are continuously looking for new colored phosphors for use in such LEDs to produce custom colors and higher luminosity.
In addition to colored LEDs, a combination of LED generated light and phosphor generated light may be used to produce white light. The most popular white LEDs consist of blue emitting GaInN chips. The blue emitting chips are coated with a phosphor that converts some of the blue radiation to a complimentary color, e.g. a yellow-green emission. Together, the blue and yellow-green radiation produces a white light. There are also white LEDs that utilize a UV emitting chip and a phosphor blend including red, green and blue emitting phosphors designed to convert the UV radiation to visible light.
One known yellow-whitish light emitting device comprises a blue light-emitting LED having a peak emission wavelength in the near UV-to-blue range (from about 315 nm to about 480 nm) combined with a yellow light-emitting phosphor, such as cerium doped yttrium aluminum garnet Y3Al4.9O12−δ:Ce3+ (“YAG”) wherein δ is typically between about −1 to 1. The phosphor absorbs a portion of the radiation emitted from the LED and converts the absorbed radiation to a yellow light. The remainder of the blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. A viewer perceives the mixture of blue and yellow light, which in most instances is perceived as a whitish-yellow light.
The blue LED-YAG phosphor device described above typically produces a white light with a color rendering index (CRI) of from about 74–82 with a tunable color temperature range of from about 4500 to 8000 K. While suitable for some applications, most users desire a light source with a higher CRI, one with a CRI similar to incandescent lamps approaching 95–100.
The limitation in CRI and CCT values for blue LED-YAG light sources is due in part to the lack of red in the device emission spectra. Red phosphors, such as CaS:Eu2+ have been added to such devices in increase the range of CCTs and CRIs available. However, these new red materials have the disadvantage of absorbing radiation emitted by the YAG phosphor, resulting in an inevitable loss mechanism due to the reduced quantum efficiency. State of the art red enhanced devices have a typical conversion efficiency of only about 70% that of blue LED-YAG devices.
It would be therefore be desirable to develop a nearly full-spectrum LED based light source capable of producing CCTs ranging from 2500 to 10000 K and CRIs from 50–99 without a significant drop in quantum efficiency compared to YAG based devices. The present invention provides a new and improved phosphor blend and method of formation, which overcomes the above-referenced problems and others.